F.3.2 Implementation-defined attributes

Analogous to pragmas, the set of attributes may be extended by an
implementation. All GNAT-defined attributes are described in
Implementation Defined Attributes, and these include several that are specifically intended
to correspond to other vendors' Ada 83 attributes. For migrating from VADS,
the attribute VADS_Size may be useful. For compatibility with HP
Ada 83, GNAT supplies the attributes Bit, Machine_Size and
Type_Class.